Optical effect structures

ABSTRACT

An optical coating structure is provided that when applied to a surface of an object to imparts a color to the object, the optical coating structure comprising: a base layer; a reflector on the base layer; and profile elements on the base layer under the reflector, the profile elements having a width and length which are each in the range of 5 to 500 μm in size, and being arranged in non-periodic manner or a periodic manner. The reflector may be a multilayer structure of alternating dielectric materials. A method of forming the optical coating structure is also provided.

FIELD

The invention relates to optical effect structures, in particular to anoptical coating structure for imparting a desired color to an objectwhen applied onto its surface.

BACKGROUND

An optical coating structure is a stack of one or more thin layers ofmaterial deposited on a substrate or object in a way that alters the wayin which the object reflects and transmits light. The thin layers aredeposited typically to a thickness of between 10 nm to 200 nm.

For example, a quarter-wave stack reflector is a well-known buildingblock of optical thin-film products. Such a stack generally comprisesalternating layers of two or more dielectric materials with differentrefractive indexes, in which each layer has an optical thickness (i.e.,the geometric thickness of the layer multiplied by the refractive indexof the layer material) that corresponds to one-quarter of the principalwavelength of reflection. Here, the wavelength of light reflected varieswith angles of incidence and reflection, thus one can observe differentcolors at different viewing angles; a visual effect known asiridescence.

In this manner, an optical coating structure can be constructed toaccurately and selectively reflect certain wavelengths of visible lightin order to impart a desired color to an object at particular angles ofincidence. Unlike paints in which the color is determined by pigments ordyes that are held together with binders, with optical coatingstructures the transition from near total reflection to maximumtransmission can take place over a very short wavelength range, enablinga precise discrimination between different wavelengths. As a result,objects coated with such structures can take on a sharp and well-definedcolor, albeit each color is observable over a narrow range of directionsonly.

It is known from JP-A-2005/153192 to provide an optical coatingstructure comprising a base structure that has been etched to provide asurface with a large number of small (less than 500 nm) crevices withsides that extend normal to the surface of the structure. On top of thebase structure is a coating made up of two polymeric layers alternatelydeposited on the substrate, one of the polymeric layers having a highrefractive index and the other having a low refractive index. The layersare deposited so that they imitate the uppermost surface of the basestructure and as a result, each layer, including the uppermost layer ofthe structure, has the same profile of crevices with sides that extendnormal to the surface of the structure.

The structure disclosed in JP-A-2005/153192 provides a chromogen inwhich the color changes with the viewing angle and provides a gentlewavelength dispersion, a deep hue and a high reflectivity. The visualeffects produced by this device are, at least in part, caused bydiffraction effects caused by the crevices formed in the top layer andother layers of the device.

EP 1923229 describes a security device which has an arrangement ofoptical elements which each comprises a stack of curved alternatinglayers of different refractive indices. This security device changes itsappearance depending on the the type of incident illumination andprovides the appearance of a single dull color over its surface whenviewed under diffuse light, giving the appearance of plastic.

WO 2011/161482 discloses an optical effect structure which comprises amultilayer reflector deposited on scattering structures which aresub-micron in size. This optical effect structure provides a brightcolor effect with minimal iridescence.

However, it remains desirable to provide an optical coating structurecapable of providing a color that is sufficiently bright and thatexhibits a minimal or limited iridescence effect, i.e., so that thecolor remains substantially the same to an observer over a broad rangeof viewing angles, whilst being relatively easy, cheap and/or reliableto produce.

SUMMARY

According to a first aspect, the present invention provides an opticalcoating structure that when applied to a surface of an object imparts acolor to the object, the optical coating structure comprising: a baselayer; a reflector on the base layer; and profile elements on the baselayer under the reflector, the profile elements having a width andlength which are each in the range of 5 to 500 μm in size, and beingarranged in non-periodic manner or a periodic manner.

According to a second aspect the present invention provides a method offorming an optical coating structure, the method comprising: providing abase layer, the base layer having profile elements thereon, the profileelements having a width and length which are each in the range of 5 to500 μm in size, and being arranged in a non-periodic manner or aperiodic manner; and depositing a reflector on the base layer.

The reflector may be a narrow band reflector. Such a reflector mayproduce reflections over a limited wavelength ranges (e.g. 50 nm orless). The reflector is, however, distorted by the underlying reliefprovided by the profile elements.

It has been surprisingly realised that it is possible to have an opticalcoating structure which provides a color that is sufficiently bright andthat exhibits a minimal or limited perceived iridescence effect withoutrequiring the profile elements to be sub-micron in size. The opticalcoating structure has a metallic, almost anodised appearance.

WO 2011/161482 discloses an optical effect structure in whichsemi-circular (in cross section) submicron base structures are present.These base structures cause the multilayer reflector (i.e. stack)deposited thereon to conform to their profile as concentric circles. Thesize of the base structures are designed so that the very top (outer)layer of the multilayer reflector would form the shape ofnear-juxtaposed semicircles in cross-section. In other words, therewould be a decreasing amount of flat area in each layer of themulti-layer reflector at an increasing distance from the base layerincluding the profile elements. Consequently, as a series ofsemicircular bumps, incident light would always strike the stack at thenormal, regardless of (or at least over a very large) angle ofincidence. If the base structures were a little larger, let alone anorder of magnitude larger, then it would be one of the lower layers thatwould satisfy this condition, and the top layer would form a series ofjuxtaposed but shallow arcs that would not present the normal positionfrom many angles of incidence. i.e. in the device of WO 2011/161482 itwas thought that the arcs at the surface layer should be semicircles forthe effect to be achieved.

It has surprisingly been found that a bright color can be provided withminimal perceived iridescence even when the profile elements are notsub-micron in size.

In the present case, the base structures, i.e., the profile elements,are not submicron and in fact are between 5 and 500 μm in size. In thiscase, the reflector has a near-identical profile to that of the profileelements. In the case of a multilayer reflector each layer may havesubstantially a near identical profile. This profile is the same as thatof the base structures/substrate. This is because the base structuresare significantly larger than the thickness of the layer(s) of thereflector. For example, the reflector may have a thickness less than 1μm, or less than 200 nm.

It has been found that those parts of the reflector that are illuminatedby incident light actually satisfy the normal condition. When theincident light is at 90 degrees to the whole structure, the tops of thepeaks (and possibly the bottom of the troughs depending on the shape ofthe profile elements) will reflect, and these will present the normalcondition. When light is at 45 degrees, only one side of the profileelements will be illuminated and reflect. However, the reflector willstill satisfy the (roughly) normal condition. It has been found thatalthough the sides are not quite flat, but roughly (and optionallysinusoidally) curved, due to global averaging by the eye of the observer(considering the size of each bump) this is averaged to a perceivedsingle wavelength of reflection.

The optical effect structures may also, when in directional light,create a color with a slight sparkling effect. This is because the humaneye may perceive reflections from individual profile elements whichcreates a ‘sparkly’ effect.

Thus, the optical effect structures can be considered as a two-partsystem utilizing nanotechnology to provide the thin films of thereflector and micro-technology to provide the optical elements of theoptical coating structure. The manufacturing processes involvemicro-structuring the base layer to create relief in the base layerbefore the thin films of the reflector are deposited.

The profile elements are shaped so as to result in a portion of thesurface, for example greater than 25%, greater than 30%, greater than40% or greater than 50% being approximately normal to the incoming lightwhen the light is incident at the normal to the plane of the surface.The profile elements are also shaped so as to result in a portion of thesurface, for example greater than 25%, greater than 30%, greater than40% or greater than 50% being approximately normal to the incoming lightwhen the light is incident at an angle of up to 45 degrees.

It has been found (as would be expected) that the change in wavelengthof reflected light over viewing angle (i.e. iridescence) is greater thanwhen the profile elements are submicron in size. However, it has beenfound that this increased iridescence is not sufficient to give a visualimpression of iridescence to a human observer. This is because of howthe reflected light is perceived by the observer. In other words,although when measured with a spectrometer there may be a shift in thewavelength of light reflected it has been found that this shift is toosmall to be detected by the human eye.

Also, it has been found that having greater than 25%, greater than 30%,greater than 40% or greater than 50% of the surface normal to theincoming light (even for angles up to about 45 degrees) is sufficientfor just the color created by the reflections at the normal to beperceived, i.e. a single rich color is observed. It has been found thatthe submicron profile elements of the prior art will provide the desiredbright color with minimal or no iridescence up to about 1 micron in sizeand that the larger profile elements, which are 5 to 500 μm in size(width and length), will provide the desired bright color with minimalor no perceived iridescence. However, with profile elements betweenabout 1 and 5 microns the effect of iridescence is perceived by anobserver.

Thus, when the submicron profile elements were increased in size it wasfound that at about 1 micron the limited iridescence effect started todisappear. It has since surprisingly been found however that withprofile elements which are about 5 microns and larger the effect oflimited or no perceived iridescence was again achieved. The opticalcoating structure of the present invention also has the advantage thatit can be easier to manufacture than an optical coating structure whichhas sub-micron scattering structures.

Viewed from another broad aspect, the present invention can be seen toprovide a method of imparting a structural color to an object byincorporating on a surface of the object an optical coating structurecomprising: a base layer; a reflector on the base layer; and profileelements on the base layer under the reflector, the profile elementshaving a width and length which are each in the range of 5 to 500 μm insize, and being arranged in non-periodic manner or a periodic manner.The optical coating structure may be applied to more than 50% of asurface of the object, 60%, 70%, 80%, 90% of the surface of the object.It may be applied to substantially the whole of the outer surface of theobject or to the entire outer surface of the object to impart a color tothat object through the structure of the optical coating structure. Theobject may be hollow and the optical coating structure may be applied toan internal surface to impart a color to the inside of the object. Thestructural color may be in place of pigments that would otherwise beused to impart color to the object.

The present invention can also be seen to provide a structurally coloredobject that comprises an optical coating structure on a surface of thatobject, the optical coating structure comprising: a base layer; areflector on the base layer; and profile elements on the base layerunder the reflector, the profile elements having a width and lengthwhich are each in the range of 5 to 500 μm in size, and being arrangedin non-periodic manner or a periodic manner.

Brief Description of Certain Optional Features

The following is a brief description of certain exemplary features whichare optional to the present invention.

The profile elements are mainly between 5 and 500 μm in width andlength. By this it is meant that the width and length of at least 60%,75%, 80%, 90% or 99% of the profile elements (by area), for example, arebetween (and including the end values of the ranges) about 5 to 500 μm,5 to 100 μm, 5 to 30 μm, 10 to 30 μm or about 10 μm.

One preferred range for the width and length dimensions of the profileelements is 10 to 50 μm, though commercial considerations may favourlarger profile elements, e.g., extending up to 500 μm (particularly, butnot exclusively, in case where the profile elements are in the form ofrecesses). This may account for 80%, 90%, 95% or 99% of the profileelements (by area).

The height of the profile elements may be between 0.1 and 50 μm, 1 to 5μm, or 2 to 3 μm. At least 60%, 75%, 80%, 90% or 99% of the profileelements (by area), for example, have a height between (and includingthe end values of the ranges) about 0.1 and 50 μm, 1 to 5 μm, or 2 to 3μm.

The height of a profile element may be the dimension of the profileelement (in a direction perpendicular to the plane of the surface of thebase layer) from the lowest point of an adjacent trough in the baselayer on which the profile elements are located to the furthest mostpoint of the profile elements from the plane of the surface. The lengthof a profile element may be the dimension of the profile element whichis largest in a direction which is parallel to the plane of thesubstrate (i.e. base layer) and the width of the profile element may bethe dimension which is parallel to the plane of the substrate andperpendicular to the length of the profile element. For example, in thecase of a perfectly conical profile element the height would be thedistance from the plane of the substrate to the peak/tip of the cone andthe width and length would both be equal to the diameter of the cone atits base, i.e. at the plane of the surface. In the case of a concaveindentation profile element, the height would be the distance from thebottom of the indentation to the top.

The combination of surface area to height measurements mentioned abovehelps to maintain the profile elements as low aspect ratio bumps orhollows. In this way a strong reflection may be maintained when viewingthe optical effect structure within a cone of 90 degrees of the surfacenormal (for light incident normal to the structure) where littleiridescence is observed. If the profile elements are made to a higheraspect ratio, then this may start to reduce the percentage “surfacearea” adding to the strong reflection since more of the profile elementarea will be made up of the lower regions of the profile elements whichare not adding to the reflection. In addition, steeper curvature in thereflector will lead to more iridescence and the possibility ofreflections becoming trapped by the profile elements.

Profile elements having a height of >5 μm may provide a good coloreffect, for example, when the same dimensional relationships (aspectratios) are maintained for profile elements having larger surface areawithin the ranges described above (i.e., larger lengths, widths,diameters, etc.). Where the manufacturing process means that the profileelements can be made to a high level of precision, this will increasethe strength of the reflection. As a result higher aspect profileelements manufactured to a high level of accuracy can produce the samestrength of reflection as some optical coating structures having loweraspect profile elements. Moreover, any loss of the optical effect whenhigher profile elements are used may also not be that significant andmay still produce an acceptable product. At least 60%, 75%, 80%, 90% or99% of the profile elements (by area), for example, may preferably havea height of <50 μm, more preferably ≤25 μm and more preferably still ≤15μm; most preferably these profile elements are of a height ≤10 μm. Insome embodiments, at least 60%, 75%, 80%, 90% or 99% of the profileelements (by area), for example, have a height in the range of (andincluding the end values of the ranges) 1 to 10 μm, more preferably 2 to10 μm.

The profile elements are introduced to the basal layer of the reflector(this is due to the profile elements being on the base layer to whichthe reflector is applied) to cause a degree of scattering in thereflector.

The base layer of the optical coating structure may be a surface of theobject to which the optical coating structure is applied to impartcolor. The object may be any article onto which a desired color is to beimparted. For example, the optical coating structure can be applied tosurfaces such as any product or device such as a plastic case for acommunication device such as a mobile telephone, watches, computers,pens, household objects, coatings for vehicles, glass or crystalornaments, glass, crystal, metal or polymer jewelry, or plastic objects,or to smaller surfaces such as flakes for cosmetics or paints. A productor device may be referred to as an object or vice versa. The opticalcoating structure may also be used in an art piece. It may be applied toceramic articles, for example, in place of a glazing. It may be used toprovide a color effect on an article of frosted glass or frosted plastic(e.g., an acrylic like Perspex), for example, a privacy screen orcolored transparent wall, or it may be used to provide a color effect ona product comprising a defined region of frosting on a transparentglass/plastic surface, for example, cabinet doors, decorative screens,etc.

There is preferably no break in the continuity of the layer(s) of thereflector and the profile elements are configured to avoid diffractioneffects.

The average period (this is the average distance between adjacentprofile elements (e.g. peak to peak)) between adjacent profilestructures may be between 5 and 100 μm, or about 25 μm. The desiredaverage period may depend on the size and/or shape of the profilestructures.

The surface of the object on which the profile elements are provided maybe flat (i.e. planar) at the macro scale (i.e. over the order ofmillimeters). Alternatively, the surface may have a curvature. It hasbeen found that the principle of reflection would break down at a ratioof the curvature of the substrate to the curvature of the profileelements of 1:5 or less. The ratio of the curvature of the substrate tothe curvature of the profile elements may be about 1:50 or greater, orabout 1:100 or greater. In some embodiments it may be desirable to formthe profile elements on a flat substrate material and then bend thesubstrate into a more complex three dimensional shape, for example, amoulding process when manufacturing a housing for an object, such as aportable communication device having bent edges. During the bendingoperation, the profile elements may become distorted into strips. Thedimensions of the distorted protrusions or recesses may still be withinthe ranges previously mentioned.

The multi-layer reflector may be applied to the substrate/profileelements prior to the bending operation or it may be appliedsubsequently. In both cases, a good color effect may still be achievedeven for the bent/curved regions of the object.

The profile elements may comprise an upper, convexly curved surface. Thecurved surface may extend symmetrically either side of an uppermostpoint.

The profile elements may be protrusions from the surface of asubstrate/base layer.

The profile elements may instead be indents (hollows) formed in thesurface of a substrate/base layer.

While the profile elements are preferably smooth, curved shapes, theymay include steps or flat sides to create the overall shape of theoutwardly extending projection or inwardly extending indentation.

The profile elements may be protrusions or indents formed by rougheningthe surface of a substrate/base layer. In one example, the roughenedsurface is a roughened glass surface. In another it is a roughenedplastic surface. Other materials, such as ceramics and metals may alsomake suitable substrate/base layer materials.

The profile elements can be any shape providing the width and length of(at least 60%, 75%, 80%, 90%, 95% or 99% of the profile elements (byarea) of) the profile element is between 5 and 500 μm in size, 5 to 100μm in size, 5 to 30 μm in size, 10 to 30 μm or about 10 μm. Profileelements in the range of 10 to 50 μm are also desirable in someconfigurations.

For example, the profile elements (whether protrusions or indents) maybe approximately conical or frusto-conical (i.e. the projections orindents may have horizontally or diagonally flattened tops). Theseprofile elements may be roughly shaped (and the elements may vary inheight, width and length between profile elements) or the elements maybe neatly shaped and be all substantially the same shape and haveapproximately the same dimensions.

The profile elements (whether protrusions or indents) may have anapproximately part-spherical surface, for example, a convex or concavesurface respectively having a substantially even radius of curvature.The curvature of adjacent profile elements may be substantially the sameor may be slightly different, for example, within 50%, more preferablywithin 25%. The approximately part-spherical surface, as well asincluding smooth curved surfaces, may also comprise small steps orterraces, when viewed in close-up detail, for example, which might beartefacts from the manufacturing process in a 3D etching or printingprocess.

The profile elements may be a series of overlapping bumps/protrusions.

The profile elements may be a series of abutting or juxtaposedhollows/indents.

The profile elements may be randomly shaped plates (e.g. tiles) whichhave raised ends which overlap an adjacent plate.

The profile elements may include a curved surface. The surface may beentirely curved and/or angled relative to the plane of the base layer(at the micron level).

The base structure may comprise flat portions. These flat portions maybe on the profile elements themselves and/or on the surface of the baselayer between the profile elements. These flat portions may have a widthdimension which is at least 0.5 μm. If present, the flat portions mayhave a width and/or length which is between 0.5 μm and 10 μm. When suchflat portions are present the optical coating may cause an increasedsparkling effect. This is because there can be a strong mirrorreflection from each flat plane. The flat portions may vary with respectto their position and orientation on a profile element.

The profile elements may have height and width dimensions which arewithin a factor of three (0.33w≤h≤3w where w is the width and h is theheight of the profile element) and/or height and length dimensions whichare within a factor of three (0.33l≤h≤3l where l is the length and h isthe height of the profile element) and/or width and length dimensionswhich are approximately the same or within a factor of three (w≤3l),two, 1.5, 1.2. The width and length of the profile elements may besubstantially the same. For example, the profile elements may have asubstantially circular cross-section (in a plane parallel to the planeof the base layer).

The profile elements (for example at least 50%, 60%, 75% or 90% by area)may have a ratio of height and width or length dimensions between 1:2and 1:100, or 1:5 and 1:50, or 1:5 and 1:10.

The protrusions on a base structure may have variable size or a range ofdifferent sizes.

The profile elements may be closely spaced or juxtaposed. For example,the maximum gap (i.e. flat base layer) between profile elements may beless than 25 μm, 10 μm, 5 μm, 1 μm, 0.5 μm or 0.1 μm. For example theprofile elements may appear to be an array of overlappingbumps/protrusions. For example, the profile elements may be in a formthat mimics the pattern made by a plurality of closely packed bubbles onthe surface of a liquid. The profile elements may look like the tops ofpeaks which are closely spaced or have been ‘pushed’ together.

The profile elements may be closely spaced so that there aresubstantially no flat portions between the profile elements, i.e. theedges of each profile element may be in contact.

The edges of the profile elements may form an angle to the plane of thebase layer which is less than 45 degrees, 30 degrees, 25 degrees or 20degrees.

The profile elements are arranged in a random or pseudo-random manner.This may be referred to as a non-periodic manner. When the profileelements are non-periodic it is possible to prevent significantdiffraction caused by the profile elements. In other words, there shouldbe no obvious periodicity.

The profile elements may also be arranged more evenly, for example,hexagonal close-packed, where the size of the profile elements issufficient to prevent the optical coating structure acting as adiffraction grating. Accordingly they may also be arranged in a periodicmanner. However, random/pseudo-random arrangements of profile elementsare generally preferred, partly from a manufacturing perspective (randomstructures can be produced easily using acid-etching processes) and alsofrom avoiding diffraction effects.

In this specification, “pseudo-random” is considered as a randomarrangement of several adjacent structures that might reveal some degreeof order over a “larger” area, such as when examined using a Fourieranalysis, so that there is some, but not exclusive, constructiveinterference of reflected light rays of the same wavelength from thelarger area). Consequently, the profile elements serve only to broadenthe angular range of the light of wavelength reflected at the surfacenormal from a reflector.

By arranging the profile elements in a pseudo-random scattering pattern,in the case of a structure with a multi-layer reflector, the opticaleffect of the profile elements on the multilayer stack will be to reducethe stack's property of color change with changing angle, so that theobject will take on a single color that is visible from a range ofangles with little or no perceived iridescence. Such a color isgenerally brighter than most pigments while possessing a subtle and richappearance that is not glossy but instead a mesmerizingly deep,luxurious matt effect suggesting the impression of solid metal. Thiscolor has the advantage that it would not fade over time when exposed tolight as occurs with pigments.

If the profile elements are too ordered, particularly at the lower endsof the size ranges, they may cause some (undesirable) iridescence(significant color change with angle through diffraction).

At any given time, the eye detects only a narrow range of the potentialangles of reflection from an object (unless extremely close to theobject), and global averaging of wavelengths gathered at the retinaoccurs within that narrow range of detection. Therefore the profileelements described here provide the visual effect of a rich, singlecolor observable over a range of angles (for instance up to 20°, morepreferably up to 45°, either side of the surface normal), i.e. withminimal iridescent/color change effect but with an appearance slightlybrighter than that of a pigment or dye. In directional light there mayalso be a slight sparkling effect which is caused because the eye canjust about perceive the reflection from each individual profileelements.

At certain wavelengths in particular, it has been found that a change ofwavelength of up to 30 nm is not perceived by the observer. Thus, whilsta small shift in wavelength may occur over viewing angle, this is not‘seen’ by a human. For example, it has been found that the human eyedoes not distinguish different shades of blue well, but it does forgreen. For example, a difference of 25 nm in wavelength in the bluerange would not be discernible, whereas a change of 5 nm in the greenrange and the hue of color may appear fairly different to an observer.This means that for certain wavelengths at least, a small amount ofiridescence may occur but this would not necessarily be perceived by anobserver.

The base layer and/or the profile elements may be formed by depositionof material such as via printing techniques, by etching (e.g.lithographic/photochemical techniques, or other known methods used onsilicon chips which form a “negative” of the base structure by removingrather than adding material) or by stamping. The base layer may forexample be fine sand-blasted or acid-etched to form the profileelements.

The profile elements may be formed by casting, printing, stamping andetching for example. The profile elements may be formed by casting thematerial of the base layer with the profile elements thereon in a moldwhich has a surface with indents or protrusions which form the profileelements. The mold surface may have been roughened such as by gritblasting to form indents and/or protrusions thereon which cause theprofile elements to be formed when the base layer is cast in the mold.The mold may have been formed by forming it on a surface that had beenprepared from bubbles blown into a material, for example, a foamed metalstructure. This would allow a base structure with a cellular structureof overlapping bumps to be cast.

The mold may be a mold with an acid-etched surface (which can varyslightly). The mold may have an average surface roughness of 1 to 20 μm,1 to 10 μm, 2 to 10 μm, >5 to 10 μm, 1 to 5 μm, or 2 to 3 μm.

For example the base layer with the profile elements thereon may be madeusing a Cell Cast method. In such a method first, a syrup of methylmethacrylate is made, which is poured between two plates, for example,one or both of acid-etched material (this may be glass which could forexample be around 3×2 m) that form a cell. A gasket (such as a PVCgasket) is then placed around the edge of this cell to keep the ‘syrup’in, and the whole cell is then placed in an oven to polymerize thematerial (so the liquid becomes a solid).

The profile elements may be formed by adding a UV lacquer and varyingmatting agents in the mix.

Thin sheets of cured polymer (for example, about 1 mm, 2 mm or 3 mmthick sheets) can be produced by pouring a liquid form of the polymersonto a mold (e.g., a glass mold) comprising an inverse of the base layerprofile elements. There may be a boarder around the edge of the mold.The liquid may be allowed to cure, e.g., at room temperature or may beactivated, e.g., by heat or radiation. The thickness of the finishedsheet may be determined by the volume of liquid poured on to a fixedarea of mold. The polymer might be polyurethane, PDMS or silicone, forexample. In the example of polyurethane, the person may have a couple ofminutes to pour the solution into the mold before it begins to harden;it may cure fully overnight. Once cured, it may be peeled from the moldwithout splitting or leaving any material in the mold. This may achievea flawless surface with a plurality of profile elements formed thereonto provide the base structures for the reflector. The sheet may then beplaced in a coating machine. A polymer sheet of around 1-2 mm thick,e.g., 1.5 mm polyurethane sheet, may provide a suitable material forfootwear such as trainers and other articles (it may provide a form ofsynthetic leather).

Preferred embodiments of the present invention may comprise the methodstep of introducing liquid polymer into a mold, curing the polymer toform a sheet of cured polymer comprising a profile elements, andremoving the sheet from the mold to provide a base layer of the opticalcoating structure. A reflector may then be deposited on the base layer.

In the case where the profile elements are indents or hollows, they maybe formed using a substrate with a plurality of holes (aperturedsubstrate), for example, a mesh. The apertured substrate, e.g., mesh,might have apertures of the order of 10-50 μm wide, though larger sizesin the range of 50-500 μm may allow other mesh or mesh-like materials tobe available. The substrate can be dipped into, spread or otherwisecoated with a liquid polymer that then dries and/or cures to form thebase layer with the profile elements. The curing may take place on asurface or possibly under the assistance of gravity, suction or airpressure, etc., so that the polymer film is pulled into the shape of ahollow between the strands of the apertured substrate, e.g., mesh. Theapertured substrate might be in the form of a fabric-like material wheredeposited strands cross one another, or it may be formed from a sheet ofmaterial having an array of slits cut into it that are expanded intoholes or by other means such as additive manufacture. Woven fabrics orother types of cloth substrates with similar aperture sizes could alsobe used in the same way. Also substrates built up from deposited fibres(fibre mats) could be used, which would have a less uniform placement ofapertures and more random aperture sizes.

Preferred embodiments of the present invention may comprise the methodsteps of providing an apertured substrate, for example, in the form of amesh, coating the apertured substrate (e.g., by dipping) in liquidpolymer, and curing the polymer to form a base layer of the opticalcoating structure.

The reflector may be a multilayer reflector which comprises layers ofhigher and lower refractive index materials.

The multilayer reflector may comprise two or more layers. For example,the multilayer reflector may comprise three to twenty layers. Theselayers may be alternate/alternating layers of higher and lowerrefractive index materials. The number of layers will determine thereflectivity of the optical coating structure. For example, twentylayers (i.e. ten pairs of layers) should achieve 100% reflectivity andthree layers (if arranged high-low-high refractive index material forexample) should result in about 60 to 70% reflectivity. The number oflayers may be 2 to 15 layers, preferably 2 to 11 layers and morepreferably 2 to 8 layers. Although in these cases the reflectivity wouldbe less than 100%, it has been found that this reduced level ofreflection may not be perceived or noticed by an observer.

It has been found that if the multi-layer reflector contains too manylayers the effect of minimal or no perceived iridescence may start to belost. This is because the uppermost layers could start to have a profilewhich does not closely match that of the profile structures. Also, thenumber of layers affects the profile of the reflection curve (increasinglayers tend to make the reflection curve narrower and therefore moresensitive).

The multilayer reflector may be a layered quarter-wave stack comprisingalternate layers of two different materials with different refractiveindices (n) but each with the same optical thickness (actual orgeometric thickness×n=1/4λ).

Alternatively, a chirped stack with dielectric layers of varyingthickness may be used. As is well known, a chirped stack can be designedto reflect varying wavelengths of light between the layers. Chirpedstructures may for instance be preferred where the desired color isgold, silver or copper.

The multilayer reflector may be a metal-dielectric reflector. Forexample, the reflector may have a metal (e.g. aluminium) coating on thesubstrate/base layer including the profile elements and one or moredielectric layers (e.g. SiO₂ which is about 200-500 nm thick). Such amultilayer reflector can provide vivid color effects for fewer layersand better uniformity and less angle sensitivity.

Alternatively the reflector may be a liquid crystal(chiral/helical-type) reflector. In this case the wavelength ofreflection would be equal to the distance of two twists in the helicalstructure multiplied by the refractive index. Such a reflector wouldprovide a degree of circular or elliptical polarisation properties inthe reflected light.

The solid liquid crystals may for example be replicated in titania.These can be nano-engineered for a wide range of resonant wavelengths.For example, the pitch may be as low as 60 nm for a circular Braggresonance at 220 nm in a Sc₂O₃ film. All colors, in a stable, solidmaterial, can be made using this method.

There are also many other ways of producing liquid crystals in solid,stable (e.g. at room temperature) form (novel liquid crystal materialsbased on the porphyrin ring structure, or nanocrystalline cellulose, forexample).

An advantage of using liquid crystals as the multilayer reflector isthat they may be tuneable, for example, through being responsive tophysical stimuli, e.g., electrical potentials, temperatures, etc., toprovide a “tuneable color”. The optical properties of the liquidcrystals may be adjustable by using transparent filaments within theliquid crystal.

When applied to an object, an optical coating structure comprisingliquid crystals may provide tuneable color which is observable over abroader angle with less color change than with current tuneable liquidcrystals.

Flat multilayers within the liquid crystal reflector may be caused toexpand and contract to alter the color imparted by the optical coatingstructure. In this way it may be possible to adjust a peak reflection ofthe reflector, e.g., by causing a shift of a peak wavelength in therange of 350 to 800 nm to cause a change in observed color. This mightbe in response to physical stimuli such as temperature and appliedelectrical potentials causing a stress to be induced in one, two orthree orthogonal directions of the optical coating structure. Forexample, the stress may be induced through one or more devices adjacentor below the optical coating structure, e.g., one or more devices havinglayers with different thermal coefficients to make them responsive tochanges in temperature or one or more piezoelectric devices which cancause strains when electrical potentials are applied.

Preferred embodiments of the present invention may include the step ofdepositing a liquid crystal coating on the base layer when depositingthe reflector. It may include the step of fabricating one or moredevices adjacent or under the optical coating structure in order toadjust a peak reflection of the reflector. The device may comprise amultilayer structure or a piezoelectric material.

Such optical effect coatings with tuneable color may have use with manydifferent objects, for example to create a camouflage effect on anobject such as a military vehicle, craft or device, or could be used inplace of high-end paint works on cars and other vehicles, e.g., for aluxury market.

When the reflector is a multi-layer reflector, the method of forming anoptical coating structure may comprise depositing a first layer of afirst reflector material on the base layer, depositing a first layer ofa second reflector material on the first layer of the first reflectormaterial, (then if more layers are present) depositing a second layer ofthe first reflector material on the first layer of the second reflectormaterial and so on to form the multi-layer reflector.

Layers of additional materials may also be incorporated into thereflector. One or more layers of different material(s) may be applied tothe uppermost layer of the reflector. For example, the method mayinclude the step of depositing a covering (i.e. protective) layer of anoptically inactive material onto the reflector. The method may alsoinclude a further step of adhering a cut crystal or a cut glass elementto the covering layer.

Thus, in some embodiments the optical coating structure may comprise anadditional coating (a covering layer) over the multilayer reflector, forexample, a resin or other flowable product applied to the multilayerreflector, and then a further, harder layer of a transparent material,such as a layer of glass, a layer of transparent ceramic material or alayer of transparent plastics may be applied over the top. If a glass(or similar) layer is glued to the multilayer reflector of the opticalcoating structure using a covering layer of refractive index matchedadhesive, then the angle of any light incident on the surface of theglass is drawn towards the normal by virtue of the refractive index ofthe coating. The steeper angle of incidence means that the color effectproduced by the optical coating structure becomes stronger for largerangles of incidence and there is less detectable iridescence.

A glass microscope slide pressed on to a refractive index matched layer(or substantially matched layer) of adhesive, for example, produces anattractive effect because the microscope slides have a particularlyflat, smooth (at the micron/sub-micron level) outer surface. Therefractive index of the adhesive (i.e., the additional coating) shouldpreferably contrast with the multilayer coating—if it substantiallymatches the first layer of the coating then it may make the first layeroptically ineffective reducing the effectiveness of the multilayercoating.

On a smaller scale, a similar method of applying a (very thin) sheet ofglass as a top layer on an additional coating (e.g., a settable acrylic)of the optical coating structure, could be used to produce materialwhich is broken up into flakes and then embedded in a transparent mediumto produce a color effect. In one example this might be a paint. Inanother it might be a gel coat or layer of lacquer.

It is also possible to dispense with the top layer of glass or similarmaterial and instead rely on a hardened surface layer of the transparentcoating material itself, e.g., in the case of a thermally or chemicallyhardened resin, which is able to fill the spaces either in the profileelements (recesses) or between the profile elements (protrusions) at thebase of the additional coating and provide a smooth outer surface at itstop,

The covering layer may be provided so as to provide a flat surface (i.e.a surface without undulations corresponding to the profile of theprofile elements). This flat surface may be parallel to the top surfaceof the base layer (ignoring the profile elements).

The covering layer may protect the reflector and prevent it fromdetaching from the base layer or becoming clogged with dirt or grease inthe gaps between profile elements. Fingerprints can cause grease layerson the surface which appear, under the microscope, as tiny oil patcheswith interference colors, affecting the desirable luxurious matt effectof the coating. The coating may also prevent scratching of the opticaleffect structure.

Without the coating material, the structure would have a mattappearance; whereas with the coating material (given a smooth surface atthe sub-micron level) the structure may have a gloss appearance. Whetherthe structure has a matt appearance or a gloss appearance after thecoating is applied may depend on the coating material and the thicknessof the coating layer. For example, a thin layer of silicon dioxide whichconfirms to the stack profile may maintain the matt appearance of thesurface.

It is important that when a covering layer is applied to the coatingthat no air is trapped, for example in the dips/troughs which formbetween the curved parts of the uppermost layer, as this will affect theoptical effect observed. These gaps should be filled with a materialwhich has a refractive index which matches the uppermost layer or thecovering layer.

The covering layer may be made of silicon dioxide. The covering layercould be up to around 1 or 2 μm thick, for example, a 1 μm coating ofSiO₂.

In general, the specific dimensions of the layers in the optical coatinglayer will vary depending on the materials of the reflector and thedesired color to be imparted. In the case of a multilayer reflector,each layer of material of the multilayer reflector may have an actualthickness of the order of 50 nm to 150 nm for producing colors in thevisible range.

For example, each layer may be about 100 nm for a red color depending onthe materials used. The optical thickness (thickness×refractive index)should equal to a quarter of the wavelength of the desired lightreflected at the surface normal (i.e. that representing the desiredcolor observed). As will be appreciated, by varying these dimensions,different colors can be produced. For example, by reducing thedimensions, lower wavelength colors (such as violet) can be produced.

The multilayer reflector may have materials and thicknesses to result ina certain wavelength of light being reflected. For example, thewavelength may be about 425 nm to 450 nm for “blue”, about 545 nm for“green” and about 680 nm to 700 nm for “red”. These values lie at thefar end for each color because they are observed at the surface normalof a flat stack, but as the angle of viewing increases (towards glancingincidence) then wavelength reflected shifts to the near (left) part ofthe spectrum.

As an example, to produce a blue color, when the stack compriseszirconium dioxide which has a refractive index of about 2.17 and silicondioxide which has a refractive index of about 1.46, the followingmultilayer reflector may be used:

1. ⅛ wave ZrO₂=31 nm

2. SiO₂ ¼ wave=93 nm

3. ZrO₂ ¼ wave=62 nm

4. SiO₂ ¼ wave=93 nm

5. ZrO₂ ¼ wave=62 nm

6. SiO₂ ¼ wave=93 nm

7. ⅛ wave ZrO₂=31 nm

It has been found that such a seven layer multilayer reflector achievesaround 90% reflectivity of a blue wavelength, which can produce a brightand vibrant color effect.

TiO₂ may also be used as a high index material layer.

Preferably the outer layer of a quarter wave stack is a high indexmaterial to provide a stronger reflection.

Different colors can be achieved by using a different number of layersand different thicknesses, for example, in a TiO₂ and SiO₂ stack. Sevenlayers in total have been found to provide good blues, violets andsilvery or greenish blues. Different colors have also been achievedusing nine layers in total, including a deep orange, a rusty red/orangeand a pale yellow.

In general, if the two materials used for the layers have a lowercontrast in refractive index, such as Al₂O₃ and TiO₂, then more layersare generally needed to achieve suitable levels of reflectivity, butsome new colors may become possible, including emerald green. However,more layers equates to more time in the coating machine which means thatthe base layer can become hotter. Depending on the material of the baselayer, in some cases this may lead to complications such as the releaseof air bubbles or melting. The time in the machine is dependent on thematerial; the deposition rate for TiO₂ is about 8 nm/minute, whereasthat for SiO₂ and Al₂O₃ is about 13 nm/minute, at an RF power of 800 W.

Due to the effect of the profile elements on the wavelength ofreflection (the sloping sides of the base structures cause reflection ofa shorter wavelength—see below), the peak wavelength of reflection isshorter than that for a flat quarter wave stack. For example, a stackoptimised at a peak reflection of 732 nm (i.e. infra-red) can provide anorange hue when coated on the profile elements.

The multilayer reflector may even comprise as few as three layers. Forexample, it has been found that three layers in a high-low-high indexarrangement can produce the blues and violets mentioned above, but theirreflection peaks are shallower and broader (they are less bright andreflect a greater portion of the spectrum, which is in turn averaged bythe eye. In this case the higher refractive index material may be theouter layer. For example the multi-layer reflector may be a layer ofZrO₂ on the profile elements, then SiO₂ and then a final layer of ZrO₂.In this case the higher refractive index material (the zirconiumdioxide) is the outer layer.

When the layers of a multilayer reflector are deposited on the baselayer that incorporates the profile elements, the reflector layers willfollow the shape of the cross section of the profile elements. This isbecause the thickness of the reflector layers is much smaller than thesize of the profile elements.

The layers will reflect incident light into a range of directions, butaveraging around 0 to 20 degrees from the normal. Therefore, alsoconsidering the global averaging of the eye, the colored appearanceproduced by the optical coating structure will be one of a single color,changing only slightly in hue with changing angle. Effectively, thiswill create a structural color that appears to be a substantiallyuniform color from all directions.

Due to the fact that the size (i.e. height, length and/or width) of theprofile elements is significantly larger than the thickness of thereflector, the reflector will conform to and follow the surface profileof the structure substrate/base layer. For example, the height, widthand/or length of (for example at least 50%, at least 60%, at least 75%,at least 90% by area of) the profile element may be at least 10 times(e.g. 10 to 100 times) larger than the thickness of the reflector.

In terms of materials, the base layer and/or the profile elements of thebase layer may comprise a transparent or black-colored material (such asblack Perspex). When the base layer is transparent it may be coated witha material which is opaque to at least visible light. This coating maybe over the whole surface of the base layer including the profileelements or the coating may be on at least the back and sides of thebase layer. This is to prevent light coming through the optical coatinglayer from the base layer. Having a black base layer or a base layerwith a black or opaque coating/backing will prevent stray light enteringthe substrate from the sides or underside of the base layer and thusallow a more brilliant color to be produced by absorbing thosewavelengths in incident light that are not desired for reflection.

It has been found that if a white or transparent substrate/base layer isused a pearly effect can be obtained. One particular application for thecoating may be on frosted glass where the reflector produces a coloreffect on the glass. The glass may be a panel or it may provide aportion of the object. The coating may also be applied to ceramics,e.g., china to simulate a glaze, or to plastics, e.g., common plasticsfor housings and articles such as acrylics.

Although the cone of reflection (i.e. the maximum angle at which noiridescence is perceived) is narrower than for prior art structureswhich are submicron, when the structures are applied to a blacksubstrate the iridescence fades with angle into black. This is adesirable visual effect which is suggestive of a luxurious surface.

The materials used in the layers of the multilayer reflector may begenerally dielectric materials such as silicon dioxide, titaniumdioxide, zinc oxide, zinc sulfide, magnesium fluoride, zirconium dioxideand tantalum pentoxide. For example, the multi-layer reflector maycomprise alternating layers of a relatively (compared to the othermaterial of the multilayer reflector) high refractive index layer suchas zinc oxide and a relatively (compared to the other material of themultilayer reflector) low refractive index material such as siliconoxide.

The covering layer may comprise silicon dioxide or other various(optically transparent) glasses. The covering layer may be made of achemical vapor deposited poly(p-xylylene) polymer (e.g. Parylene).

The various layer(s) of the reflector may be produced and applied ontothe base layer using a number of fabrication steps well-known to thoseof ordinary skill in the art such as printing, ion beam deposition,physical vapour deposition, chemical vapour deposition, molecular beamepitaxy sputter coating, dip and spray coating or self-assembly methods.

During manufacture of the optical effect structure, after the base layerwith the profile elements has been formed, if the reflector is notapplied immediately, it may be covered with a temporary protective film.This film may be removed immediately prior to application of thereflector. This temporary protective coating can prevent damage to theprofile elements. In particular it may prevent dirt or grease cominginto contact with the profile elements which may be difficult to cleanprior to applying the reflector.

As previously mentioned, at any given time the eye detects only a narrowrange of the potential angles of reflection from an object, and globalaveraging of the wavelengths gathered at the retina occurs within thatnarrow range of detection. As known, the color of light observed at aparticular angle will depend on the optical distance of each layerthrough which the light travels. When a multi-layer reflector is viewednormal to the underlying base layer the light will travel a distancethrough each layer which is equal to the thickness of each layer. As aresult, due to interference effects and global averaging in the eye, thecolor of light observed normal to the coating will be the color of lightwhich is determined by the geometrical thicknesses of the layers and therespective refractive indexes of the layers. In a normal quarterwavelength stack (i.e. with no underlying profile elements), when viewedfrom an angle, the light detected by the eye at that angle will havetravelled slightly further through each layer (a distance greater thanthe thickness of each layer) and thus the optical thickness travelled bythe light rays will be larger. Light with a longer wavelength will beobserved from the broader viewing angles giving rise to iridescence.

However, the presence of the underlying profile elements causes thelayers to vary in angle relative to the plane of the substrate/baselayer (as they follow the profile of the profile elements). Consequentlyover a broader range of viewing angles, a significant proportion of thereflector layer surfaces producing the observed reflections will beorientated more to the observer in a way that also substantiallymaintains the intended thicknesses in the layers of the reflector, i.e.the normal position. As a result, the color observed by the eye overthat broader range of angles is relatively constant.

BRIEF DESCRIPTION OF FIGURES

Certain preferred embodiments of the present invention will now bedescribed in greater detail by way of example only and with reference tothe accompanying drawings, in which:

FIGS. 1A and 1B show a plan view and side view of an optical effectstructure;

FIGS. 2A and 2B illustrate the approximately ‘normal’ portions of theoptical effect structures;

FIG. 3 shows an electron micrograph of the optical effect structureshown schematically in FIGS. 1 and 2;

FIGS. 4, 5 and 6 show alternative optical effect structures; and

FIGS. 7A-7D illustrate schematically the operation of reflections withthe optical coating structure;

FIGS. 8A and 8B illustrate scanning electron micrographs of the twotypes of base structure, namely profile elements in the form of convexlycurved projections and as concave indentations (one is the inverse ofthe other);

FIGS. 9A and 9B shows scanning electron micrographs illustrating across-section of an indentation-type base structure, as formed in a(approximately 20 μm thick) lacquer applied to the surface (FIG. 9B is ahigher magnification of the central part of FIG. 9A);

FIGS. 10A and 10B illustrate theoretical reflection profiles of a threelayer stack (FIG. 10A) and a seven layer stack involving SiO₂ and ZrO₂where ZrO₂ forms the innermost and outermost layers (FIG. 10B);

FIG. 11 illustrates measured transmission curves for an “electric blue”optical coating structures sample;

FIGS. 12A and 12B illustrate scanning electron micrographs (samemagnification) from different batches of production;

FIGS. 13A and 13B illustrate scanning electron micrographs of twoindentation type base structures coated with three layers (blue) in asputter coating machine;

FIGS. 14A and 14B illustrate a scanning electron micrographs showingdamage to a coating during an abrasion test of a sample optical coatingstructure;

FIG. 15 shows the emission spectrum of a white lamp used in thereflectivity measurements of FIGS. 16A to 20B;

FIG. 16A shows a series of reflection spectrums received from a blueoptical effect structure having indentation-type profile elements, andFIG. 16B shows reflection spectrums received from a blue glass opticaleffect structure having indentation-type profile elements;

FIGS. 17A and 17B show reflection spectrums from a blue optical effectstructure (having indentation-type profile elements) at 45° and atnear-normal respectively;

FIGS. 18A and 18B show reflection spectrums from a crimson opticaleffect structure (having indentation-type profile elements) at 45° andat near-normal respectively;

FIGS. 19A and 19B show reflection spectrums from a glass optical effectstructure (having indentation-type profile elements) at 45° and atnear-normal respectively;

FIGS. 20A and 20B show reflection spectrums from a blue optical effectstructure (having projection-type profile elements) at 45° and atnear-normal respectively;

FIG. 21 illustrates schematically an optical coating structurecomprising an additional coating layer;

FIG. 22 illustrates schematically an optical coating structurecomprising an additional coating and a top layer;

FIG. 23 is a schematic representation of an optical coating structurewhere a top layer comprises a cut crystal;

FIG. 24 is a schematic representation of an optical coating structurewhere a top layer comprises a convex shaped element;

FIG. 25 is an exemplary flow diagram of a production process for apreferred optical coating structure;

FIG. 26 is a flowchart illustrating steps for providing a base layer ofan optical coating structure;

FIG. 27A is a schematic representation of an apertured substrate in theform of a mesh;

FIG. 27B is a schematic representation of an apertured substrate in theform of a mesh coated in a polymer; and

FIG. 28 is a flowchart illustrating steps for an alternative way ofproviding a base layer of the optical coating structure

DETAILED DESCRIPTION

The color effect produced by the present optical coating structure canbe explained using a unique combination of nanophotonics and geometricoptics. A thin film stack may be used to generate color (in whitelight/sunlight) while juxtaposed, shallow arcs, one or two orders ofmagnitude larger, can be used to form a base that causes many tinyreflections over the surface that do not present significantly differentangles to the incident light over most angles of observation. The globalaveraging function of the eye reduces the effect of “variations” inwavelength giving the impression of deep, luxurious, single color.

The thin film stack can take the form of, for example, quarter-wavelayers (usually several layers) or a metal plus dielectric (two-layer)arrangement, but the base layer and reflector layers are generally flatat the micron scale. The visual effect is that one (“peak”) wavelengthappears to dominate the reflection from white light at each angle theoptical coating structure is viewed from—in essence the observer sees astrong color corresponding to that wavelength. The reflection appears asa bright beam, although a different wavelength can dominate as the angleof incidence/observation changes, for example, resulting in a brightspectrum with different colors seen from different directions at thesteeper angles.

FIGS. 1A and 1B show an optical coating structure that when applied to asurface of an object imparts a color to the object. The optical effectstructure has a base layer with profile elements 2 thereon. The profileelements 2 have a width and length which are each in the range of 5 to100 μm in size, and are arranged in non-periodic manner (though they mayalso be arranged in a periodic manner). The heights of the profileelements 2 are in the range of about 1 to 10 μm (in some embodimentsthey may be in the range of 1 to 5 μm or >5 μm and 0 μm, for example,2-10 μm). The profile elements 2 have valleys/troughs 4 between theraised profile elements 2 as shown for example in FIG. 1B.

A reflector is provided on the base layer. However, because thereflector is significantly thinner than the size of the profile elements2 it is not shown in the Figures as it is not visible at this scale.

The reflector is a narrow band reflector. It may be a multilayerreflector which comprises alternating layers of higher and lowerrefractive index materials. It may comprise a quarter wave stack, whereeach layer is a quarter of the desired wavelength of reflection in“optical thickness” (actual thickness×refractive index). Preferably twomaterials are involved and are deposited alternately, e.g. SiO₂ as thelow index layer, and TiO₂ or ZrO₂ as the high index layer. Seven layersin total (high, low, high, low, high, low, high) are sufficient toproduce a bright and vibrant color effect for some colors, for example,up to 90% reflection may be achieved. Ideally, the outer layer should bethe high index material, since this provides a stronger reflection. Notethat for some stacks, the innermost and outermost layers may be ⅛^(th)wavelength in optical thickness.

The thin films of the multilayer reflector can be deposited usingdip-coating, vacuum coating, sputter coating, plasma coating,liquid-crystal chiral methods or block copolymer methods, etc., in amanner that is standard for optics manufacture (e.g. for filters).Plasma coating in particular is a high-level technique that providesparticularly uniform coating partly through avoiding very hightemperatures. Materials such as ZrO₂, TiO₂, Al₂O₃ and SiO₂ adhere wellto many substrate materials, although some materials (such aspolycarbonates) may require the addition of an “attachment” layer. Thethin films can be deposited as a quarter wave stack to produce specificcolors.

Another form of thin films may be a metal and dielectric coating (i.e.two layers only) approach. The thickness of the dielectric (outer) layer(e.g. SiO₂) will determine the wavelength reflected—it should be half awavelength thick in optical thickness. Silver may be made by thismethod, and an attractive emerald green and crimson are also possible.Colors tend to be less bright and vibrant, and more subtle, and tend tochange vary less with changing angle of viewing.

As shown, the profile elements 2 in the embodiment of FIG. 1A form acellular structure of overlapping/juxtaposed protrusions with troughs 4therebetween. By contrast, the profile elements 2 in the embodiments ofFIG. 7D and FIG. 8B form a cellular structure of overlapping/juxtaposedhollows or indents with ridges 5.

Thus the substrate or base layer can have the form of micro-projections(positive), involving juxtaposed, shallowly curved bumps, ormicro-indentations of the inverse (negative) shape, which form the basestructures or profile elements. The profile elements may be evenly (e.g.hexagonally close-packed) or randomly shaped and vary inwidth/length/diameter from about 10-50 μm, and in height between about 1and 10 μm. The profiles and dimensions need to be selected carefullysince if the curves are too deep, the color effect will be lost (ormight change to a different effect).

The role of the profile elements is to cause the thin films deposited onthem to take on their profile throughout the stack. The shallowundulations in the thin-film stack have the effect of causing the samepeak wavelength to be reflected over a broad range of angles ofincidence/observation. When the effect of global averaging of the eye istaken into consideration as well (which negates the effect of “stray”reflections), causes the optical coating structure to appear as a singlecolor from a wide range of directions, such as within a 90 degree conecentred at the surface normal. Beyond this range, the color will beginto change to colors corresponding to shorter wavelengths in thespectrum. For example, blue will eventually give way to violet. This maybe considered a positive feature, since it signifies that somethingother than pigments are involved and creates the impression of a newoptical effect. The optical coating structure appears particularlyintense but not shiny—rather a mesmerizingly deep, luxuriously matteffect. When it is applied on a plastic substrate (i.e., the base layeris a plastics material), the optical coating structure can provide anappearance that suggests the presence of a solid metal.

The profile elements 2 vary in size and at least 80% (by area) of theprofile elements 2 have a width and length which are each in the rangeof 5 to 100 μm in size and a height in the range of 1 to 5 μm.

As shown in FIG. 2A when light is incident normal to the base structureabout 50% of the surface (i.e. the reflector which matches the profileof the profile element) is approximately normal to the incoming light.As shown in FIG. 2B even when light is incident to the base structure atan angle (for example up to 30 degrees) a relatively large percentage(such as at least 40%) of the surface is approximately normal to theincoming light. As a result, the color which is perceived by a human canbe the same even when the structure is viewed at an angle of up to 30degrees from the normal.

Due to the way in which a human processes wavelengths of light, havingat least 40% of the surface reflecting approximately the same wavelengthwill result in a single pure color being perceived by the observer.

When the narrow band coating is being viewed at a shallow angle to thenormal (e.g. ±30 degrees) the optical thickness of the layers is largerleading to a shifted reflection. Due to the nature of the underlyingprofile elements some of the observed rays will always have a componentof the shifted wavelengths, and as a result, it is necessary whendesigning or forming the optical effect structures to have to choose amodified wavelength (by altering the reflector) to reach the desiredperceived wavelength.

FIG. 3 shows an optical electron micrograph taken of an optical effectstructure. The magnification was 366×, the EHT was 20.00 kV, the signalis received from a backscattered detector (BSD), the pressure was 20 Pa,the working distance (WD) was 15 mm and the spot size was 510. Thisimage clearly shows the cellular structure.

FIGS. 4, 5 and 6 show alternative embodiments. As shown in FIG. 4 theprofile elements 2 may have varying heights. As shown in FIG. 5 theprofile elements 2 may be equal sized elements. As shown in FIG. 6 theprofile elements 2 may have flat portions which may be at varying anglesto the normal. These flat portions may result in the optical effectstructure having an increased ‘sparkly’ effect.

FIG. 7A illustrates typical reflections of light rays from smooth andrough surfaces. The smooth surface provides mirror-type reflections; therough surface provides random reflections.

FIG. 7B illustrates how the reflection of a “point” light source isobserved in a curved, smooth surface, in this case a sphere. Thereflection is broadened by the curvature. If multiple sections ofspheres are close packed, the global reflection could be made to coverover half of the entire surface.

FIGS. 7C and 7D are schematic illustrations to indicate the scale of thethin-film layers compared with the two forms of base structures formingthe profile elements (an example of micro-projections above in FIG. 7Cand an example of micro-indentations below in FIG. 7D), incross-section, and their effect on light incident from differentdirections. The reflections are broadly similar to those seen in FIG.7B, but instead originate from multiple profile elements; similar towhere multiple sections of spheres are close packed. The presence of athin-film reflector causes a change in color. However, as the wavelengthof a reflection from a thin-film reflector will vary with angle ofincidence, it means that the thin-film reflector requires a shallowercurvature than occurs in the example of the spheres in order to preventa unidirectional light from receiving a wide range of angles ofincidence. Thus the height of the profile elements is preferably 0 μm.

As with the example of the sphere in FIG. 7B, the optical coatingstructure can produce an effect where a large (for example, over 50%)proportion of the surface area appears illuminated to the eye at anymoment, although in addition the reflection will appear colored.

To produce the brightest colors, the substrate should be black andopaque, in order to prevent back-reflection of any incident light notreflected by the thin films (back-reflection would dilute the reflectedrays which have been selected for by the thin films).

Since the optical coating structure contains no pigments, it will neverfade. It is also extremely thin, for example, just 150-500 nm inthickness (or a few microns if the base structures providing the profileelements are also considered, which could be “built in” to any product).

The optical coating structure is made by forming the base layer with theprofile elements already present (for example, by moulding, stamping,printing, etc.), or by forming the profile elements on an alreadymanufactured base layer (for example, by acid etching, stamping,printing, etc.). The multiple thin-films (or alternatively chiral,liquid crystal-type structures) are deposited on top.

The profile elements of the base layer are preferably made by acidetching, to produce a suitably sized and shaped, random topography. Thesurface of a product can be acid etched or (more commercially) the innersurface of a mould or die of a product can be acid etched. For example,glass can be acid-etched and this can be used to form some or all of amould or die for a plastic material, for example, a thermoplasticmaterial such as an acrylic sheet, to leave the inverse pattern on thesurface of the finished acrylic product. The acid-etching technique isrelatively cheap making it particularly attractive from a commercialpoint of view

Alternatively, a lacquer coating process can be used. Any product can becoated in a suitable lacquer, for example, into which the mould/die canthen be applied (for example, in a stamping process). As another option,the optical coating structure could take the form of self-adhesive thinsheets which are then attached to the surface of a product (this can becarried out on a reel-to-reel machine, using a PET or PMMA substrate forexample). Alternatively, regularly-shaped base structures can be made inthe form of a mould through Nano Imprint Lithography or by using someother three-dimensional printing methods. The profile elements also liewithin the (size and shape) fidelity limits of “standard industrial”injection moulding and blow moulding, and vacuum forming processes.Accordingly, the profile elements could be reproduced on the internalsurfaces of industrial moulds so that they become incorporated into theformed products. The formed products can then be coated. The acidetching technique can be used to form the master dies for such injectionmoulding, blow moulding or vacuum forming machinery.

In another embodiment, the hollows can be produced by dipping a veryfine polymer mesh (e.g. a mesh with holes in the order of 50 μm wide)into a liquid polymer, then removing the mesh so that as the liquidpolymer cures, it adopts a curve (dips) in the spaces between thestrands. This may be through gravity and/or molecular forces. Molecularforces may assist by forming meniscuses (providing steeper sideportions) where the polymer film attaches to the strands of the mesh anda flatter central region of the hollow where the meniscuses join in thecentre of each aperture.

The mesh preferably has apertures of the order of 10-50 μm wide, thoughmay have apertures anywhere in the range of 5-500 μm. The mesh may beformed as strands of material, e.g., strands of polymer, which crosseach other, as an expanded mesh where slits are formed in a sheet, e.g.,cut by a laser, and the sheet is then pulled to expand the apertures andform an expanded mesh, or by other means.

In place of a mesh, a woven fabric or cloth substrate can be used. Thefabric or cloth can have apertures in the same ranges as above.Similarly a cloth-like material could be built up from deposited fibreswith apertures in the same ranges. This would provide apertures with amore random distribution of sizes and spaces. In another embodiment amesh substrate with a pseudo-random distribution of apertures within thesize ranges mentioned above could be produced by additive manufacture.In another embodiment the mesh could be formed through removal ofmaterial e.g., by a laser cutting process, a photo-resist process, or anetching process to produce a pseudo-random distribution of apertures.

The mesh or apertured substrate can be dipped, spread, sprayed orotherwise coated with a liquid polymer to provide a film extendingacross the apertures. The liquid polymer can then be dried and/or curedto form the base layer structure for the reflector. A film of polymermay be offered up and adhered to the mesh or aperture substrate and thehollows induced as the polymer cures. Gravitational and molecular forcesmay be sufficient to create the hollows while the polymer cures. Ifdesired, assistance may be provided through surface pressures, forexample, through the pressure of air or a gas directed at the surface,the weight of a liquid or particles resting in the hollows as thematerial cures, or through reduced pressure below the hollows, to helpinduce curvature in the hollows.

The apertures in the substrate are bridged by the film to form thehollows or pits. The strands of the mesh may remain visible. Preferablythe strands are covered to present just one material and aid coatingwith the reflector. The liquid polymer could form a thin layer justcovering the back of the mesh or a thick layer, where the mesh is onlyvisible at the surface. The mesh or apertured substrate may then beapplied to the surface of an article to provide the base layer structureand the multilayer reflector applied to impart color to the article.

The mesh or apertured substrate material and/or the polymer materialshould preferably be dark or black to help intensify the coloredreflection. However a transparent version may also provide utility in anoptical coating structure. It may for example produce pearlescenteffects once coated with a multilayer reflector.

The finished product could be formed as a thin, colored material withcommercial applications such as a “synthetic leather” etc. Theunderlying mesh or aperture substrate may provide additional strengthfor a flexible material and may help to guard against stretching etc.

Examples of polymer materials suitable for the base layer structureinclude PDMS (polydimethylsiloxane), polyurethane and silicone. The meshsubstrate may be made from the same polymer material. Example meshthicknesses that have been shown to produce useful structures have beenin the range of 0.5 to 1 mm, more particularly 0.8 mm. Sheets having aplurality of profile elements in the form of recesses can be generatedeasily by this technique, the sheets being preferably between 1 and 3 mmthick. These can be coated with a reflector to form the optical coatingstructure. The coated sheet can be useful in the production of footwear,bags, wallets, covers, vehicle upholstery, etc., where the optical coloreffect is used to create a desired color in place of pigments.

FIGS. 8A and 8B show scanning electron micrographs of the two main typesof base structures for the optical coating structure. The micrograph ofFIG. 8A is at a magnification of approximately ×536 and shows aplurality of convexly curved projections. In the image, it is possibleto make out flattened tops to some of the projections. These areimperfections caused by inaccurate removal of the mould, and may addsome “sparkle” to the optical effect created by the coating structure.FIG. 8B is at a magnification of approximately ×478 and shows aplurality of concave indentations which provide the profile elements forthe optical coating structure. The base layer of FIG. 8B is essentiallythe inverse of the base layer of FIG. 8A.

Some forms of acid etching may cause the curved projection-type profileelements. Other forms, such as hydrofluoric acid (“HF”) etching (e.g.using 7:1 or 20:1 buffered hydrofluoric acid), involve a “2D” etchingprocess, and so several steps are required to build a 3D structure.Here, at each step the acid dissolves the substrate material to ashallow depth, leaving sloping sides and a flat base (i.e. inverted,trapezoid in cross section). Where a large flat base remains after acidetching, this will cause a mirror reflection and appear as a “sparkle”amidst the otherwise matt effect.

However, there are additional aspects to how the profile elements areformed via acid etching, which play greater or lesser roles depending onthe precise etching method. Self-organization can lead to the formationof 3D islets at the 20 μm scale (the islets may comprise straight,sloping sides forming the macroscopic shapes of the indentations);additionally, elastically-deformation forces, and the action ofdefectively-deformative and capillary-fluctuation forces may also beconsidered. Certainly, it is known that self-organizing processes thatlead to nanostructuring can occur spontaneously on surfaces undercertain macroscopic conditions.

When comparing projection and indentation type profile elements used tocreate the base structures, both types of profile elements producesimilar, attractive visual effects. There are some differences, however.The projection type profile elements, in general, cause a less sparklyeffect (and, subjectively, a possibly less-bright appearance), but tendto cause a more pronounced change in hue with changing angle.

By way of example, the profile elements may have a height (or “surfaceroughness”) of (usually) 2-5 μm (typically 5 μm). If they are mouldeddirectly into a product, then this is their actual height; if a layer oflacquer is first applied then this adds an additional 2-5 μm (minimum)to the device (i.e. the lacquer with profile elements may be around 6 μmthick at its thickest point).

The layers of the reflector may be each around 870 nm in actualthickness. Therefore, two layers add 1640 nm (0.164 microns) on to theprofile elements/lacquer base layers, while seven layers add around 500nm (0.5 μm) on to the base layers.

Consequently, the “thickness” of the optical coating structure dependson how the base layers are produced and considered. If they are mouldeddirectly into a product, then they could be considered to have a heightof either zero or 2-5 μm (or half of this (i.e. mid-height)). Then themultilayer component will add an additional thickness of around 0.164 or0.5 μm.

When considering manufacturing tolerance and which part of themanufacturing process could cause a change in visual appearance, the twocomponents of the optical effect structure should be consideredseparately.

The profile elements, in terms of optical components, these structuresare comparatively large, shallow, scalloped projections or indentations,preferably around 10-50 μm wide (preferably variable and randomlyarranged, e.g. as shown in FIG. 8A or 8B). Due to such random variationin sizes, tolerance to manufacturing variation and imperfections ishigh. However, in the case of the projection-type, it is more importantfor the curvature to be even or substantially even. If flat regionsoccur (for example, as a result of a fault in the moulding process) thenthe degree of sparkle will increase, as a result of introducing tinymirrors.

In practice, the indentation-type base structures, for example, made viaseveral steps of HF etching, can appear as different colors whenexamined at 200× magnification. There are flat areas, such as at thebase of each “pit” or “indent” and at the raised areas between pits(indents), or vice versa if the inverse structure is made via moulding.These appear as a different color of longer wavelength than the slopingregions (the sides of the pits/indents). For example, the flat regionscan appear cyan while the sloping areas appear violet. To the unaidedeye, these colors are combined and averaged to appear as a single hue(e.g. blue). However, if the flat regions are relatively large (e.g.more than 20 μm), they can be observed by the unaided eye and provide amirror-type reflection, appearing as a “sparkle”. A high frequency ofthese “defects” leads to a sparkly effect of the color device underdirectional light. This may or may not be desirable, depending on thespecific color and application.

FIG. 21 illustrates an optical coating structure comprising a base layer10 provided with profile elements 11 in the form of a pluralityrecesses. Onto the surface of the profile elements 11 is deposited amultilayer reflector 12. The optical coating structure also comprises anadditional coating 14 applied to the surface of the multilayer reflector12. The additional coating 14 may comprise a thermally or chemicallysettable material, for example, a polymeric material such as a resin. Itmay, for example, comprise an acrylic material that can be set in situ.

With a suitable choice of refractive index, the transparent material canbe selected to modify the angle of incident light rays as they approachthe multilayer reflector 12, so that they are shifted closer to thenormal as they descend through the material of the additional coating 14(the angle of incidence may shift from θ to the θ′ as shown in the FIG.21). In this way, the color effect described above becomes morereliable, even at the larger angles of incidence or viewing, because ofthis shift towards the surface normal; it reduces the tendency to createiridescence at larger viewing angles.

FIG. 22 illustrates a similar optical coating structure comprising a toplayer 16 of a smooth, hard transparent material such as a glass sheet,which has been applied to the additional coating 14. Other materialswith similar optical, refractive and mechanical properties to glass willalso be suitable, for example, certain ceramics and plastics. The toplayer 16 may itself comprise further coatings (not shown) such asanti-reflective, scratch resistant or colored coatings, as desired.Similarly, the additional coating 14 may comprise an initially flowablematerial that takes up the shape of the profile elements 11 and adheresthe top layer 16 to the multilayer reflector 12. The additional coating14 may be chemically or thermally cured. In addition, it is preferablyrefractive index matched to the top layer 16 so that the two layers 14,16, optically, act as one. In the case of a glass top layer 16, theglass will provide scratch resistance to the optical coating structure.

FIG. 23 illustrates a variation on the FIG. 22 embodiment where theplanar top layer 16 has been substituted for a layer of cut crystal 16.Light enters the crystal at different angles through the differentcrystal facets 17 which might have a dimension x (the figure isschematic and not to scale—in practice, the facets 17 of the cut crystal16 may be many magnitudes larger than the size of the profile elements12 formed in the base layer 10). Again the underlying additional coating14 should preferably be refractive index matched to the refractive indexof the cut crystal 16. Slightly different hues may be seen in thedifferent facets 17 of the cut crystal 16 by an observer.

FIG. 24 illustrates a further variant on FIG. 23 where the cut crystaltop layer 16 has been substituted for a rounded element, for example, aconvex-shaped piece of glass 16 (e.g., a dome shaped element) or othersuitable transparent material.

While FIGS. 21 to 24 show profile elements 12 in the form of recesses,the profile elements 12 could also be formed as protrusions.

FIG. 25 shows a flow diagram of possible process steps during themanufacture of the optical coating structure. At step 20, a base layeris prepared. This may comprise cutting a blank to a particular sizeand/or treating it for the subsequent steps. The base layer may be aplanar expanse of material at this stage. Then the profile elements,which may be recesses or projections, are formed on or in the base layerat step 22. This may be through an etching, deposition, moulding,stamping, printing or other suitable process. While the base layer isflat, this may make the formation of the profile elements easier. Atstep 24, the base layer may then be bent to shape, for example, in amoulding operation to form an edge of a housing. The profile elementsmay then be coated at step 26 with a multilayer reflector if they arenot already coated. The multilayer reflector may comprise a quarter waveplate reflector, for example, any of the above reflector structures. Anadditional coating, for example, a layer of curable polymeric materialmay then be applied to the multilayer reflector at step 28, and then atop layer applied to the additional coating at step 30. If desired,further coatings may be applied to the optical coating structure at step32.

FIG. 26 is a flowchart illustrating steps for providing a base layer ofthe optical coating structure according to one embodiment. In step 34,an apertured substrate is provided. This may be in the form of a mesh.The apertured substrate is then coated with liquid polymer 36. In oneexample, a mesh is dipped into liquid polymer. The polymer is cured instep 38 to form the base layer. Once cured, the polymer sheet, whichstill comprises the apertured substrate, can be introduced into acoating apparatus and a reflector can be deposited on the base layer 40.

FIG. 27A is a schematic representation of an apertured substrate in theform of a mesh 42 for use in the method. The mesh 42 includes apertures44. The mesh 42 is dipped into liquid polymer and cured to form a sheet46 of cured polymer comprising profile elements in the form of recesses48. The polymer stretches across the apertures 44 between the strands ofthe mesh 42. As it cures, the polymer is pulled into a concave shapewithin the apertures 44 to form recesses 48 as shown schematically inFIG. 27B. The sheet 46 forms the base layer, onto which the reflector isdeposited to provide the optical coating structure.

FIG. 28 is a flowchart illustrating steps for an alternative way ofproviding a base layer of the optical coating structure. In thissequence, a mold is provided in step 52. Polymer is introduced into themold in step 54. The polymer may be poured into the mold, or it may beintroduced as a powder which is melted in the mold. The mold ispatterned with an inverse of the intended profile elements. The polymeris cured in step 56 to produce a sheet of cured polymer comprisingprofile elements on a surface. The cured may be peeled from the mold toremove it from the mold in step 58. This provides the base layer of theoptical coating structure. A reflector may then be deposited on the baselayer in step 60 on the surface comprising the profile elements.

Test Results

FIGS. 9A and 9B shows scanning electron micrographs illustrating across-section of an indentation-type base structure, as formed in a(approximately 20 μm thick) lacquer applied to the surface. FIG. 9B is ahigher magnification of the central part of FIG. 9A (FIG. 9A is at amagnification of approximately ×595). In FIG. 9A, the black triangularshape (bottom left corner) is the scanning electron microscope stub (notpart of the sample). The sample is grey in the image. It comprises apiece of plastic sheet, in this case PMMA (acrylic), but could also bePVC or other suitable plastics. In the image, it is about 100 μm thick.On top of this is provided a layer of lacquer which is about 20 μm thickand contains the concave hollows. The boundary between the PMMA andlacquer forms the diagonal line visible in FIGS. 9A and 9B. In thecentre of FIG. 9B there is an overly deep hollow (a defect) that almostreaches the depth of the lacquer.

FIGS. 10A and 10B illustrate theoretical reflection profiles for a threelayer stack and a seven layer stack respectively. The layer materialsare SiO₂ and ZrO₂ and ZrO₂ forms the innermost and outermost layers. Asindicated above, due to the effect of the profile elements on thewavelength of reflection (e.g., the sloping sides of the base structurescause reflection of a shorter wavelength), the peak wavelength ofreflection is usually shorter than that expected for a flat quarter wavestack. For example, a stack optimised at a peak reflection of 732 nm(i.e. infra-red) can provide an orange hue when coated on the profileelements. Accordingly a step in the manufacture of the optical effectstructures may include adjusting the thicknesses of the layers tocompensate for the shift in the peak reflection to shorter wavelengthsin order to produce a reflection at the desired wavelength from theoptical effect structure.

FIG. 11 shows measured transmission curves at normal incidence and at10° from the normal respectively for a sample optical coating structurehaving an “electric blue” color.

FIGS. 12A and 12B illustrate scanning electron micrographs of twoindentation type base structures coated with three layers in a sputtercoating machine to produce a blue color effect. The base layers of theoptical coating structures were formed using different equipment bydifferent operators. The base layers were then coated in the samemachine to determine whether variance in manufacturing processes mightadversely affect the optical coating structures.

For the indentation-type base layer structures, while there was a widevariation in size and shape of the profile elements, no significantdifferences were observed in the optical effect generated, indicatingthat for these optical coating structures there can be considerableallowable variation in manufacturing. The samples appearedindistinguishable in color and matt effect by the unaided eye, althoughthe degree of sparkle can vary. On closer inspection in a scanningelectron microscope, there was evidence of damage during handling asillustrated by the few black regions in the micrographs. However, nodifference to the overall colored appearance of the optical coatingstructure was observed.

Multilayer coatings are standard for the optics industry. They are oftenused in high-tech applications, e.g. for spectrally tuning lasers, wherehigh accuracy is necessary. As the optical coating structures areintended to be viewed by eye for their color effect (far lower fidelity)rather than a machine reader, manufacturing variance is unlikely toproduce any perceived differences in color. The complete manufacturedoptical coating structures also were found to match the theory extremelywell, in terms of the measured wavelength and the color observed at thenormal to the samples.

Commercial coating machines are known to reproduce coatings perfectly ifthe machine parameters are set the same. This was tested on base layersproduced in acrylic for coatings at 430 nm. Samples produced indifferent coating runs appeared identical to the unaided eye. Also,different batches of coating materials are known not to affect thecolor, because the coating materials are accurately manufactured.

Thus, repeatability can be achieved through following the same designand calibrating accordingly to ensure the same results. Once set up, thecoating machine is known to run repeatedly to process all the runsneeded to complete a batch.

Table 1 below lists the results of visual assessments on a number ofoptical coating structure samples.

The sample tested produced an “electric blue” (430 nm) color effect whenviewed by eye (see FIG. 11). The optical coating structure comprised 7layers in total of SiO₂ and TiO₂. The measurements in the table aregiven to the nearest 5 degrees.

TABLE 1 Observed colour effect in an “electric blue” optical coatingstructure sample. Observed Colour Effect View- ing Color Type of Lightangle observed Appearance Diffuse Light (white room lit 0-45 vibrant,structural with bright sunlight, equally degrees: electric blue coloureffect scattered) 45-75 flat, deep blue pigment effect degrees 75-90flat, deep pigment effect degrees violet Directional light (bright, 0-45vibrant, structural summer sunlight at 5 pm, with degrees electric bluecolor effect sun at 45 degrees in sky): 45-75 flat, deep blue pigmenteffect Viewing in transverse plane to degrees that of the sunlight 75-90flat, deep pigment effect degrees violet Directional light (bright, 0-45vibrant, structural summer sunlight at 5 pm, with degrees electric bluecolour effect sun at 45 degrees in sky): 45-75 flat, deep blue pigmenteffect Viewing in plane of the degrees sunlight, in mirror reflection75-90 flat, deep pigment effect degrees violet Directional light(bright, 0-20 vibrant, structural summer sunlight at 5 pm, with degreeselectric blue colour effect sun at 45 degrees in sky): 20-60 flat, deepblue pigment effect Viewing in plane of the degrees sunlight, inretro-reflection 60-90 flat, deep pigment effect degrees violet

While Table 1 shows the results of viewing one optical coating structuresample where the sample was flat, it should be borne in mind that thesize and shape of the object colored by the optical coating structuremay also affect the psychological interpretation of the color.Additionally, each hue may vary considerably. It is thought that bluesappear in the mind of the observer to change color less than other partsof the spectrum, since a mid-blue changing to dark blue then violet willnot appear as dramatic as a yellow changing to green, for example.

Table 2 below lists a number of peak wavelengths and colors that havebeen achieved for the described optical coating structures.

TABLE 2 peak wavelengths and observed colours in optical coatingstructure samples. Peak wavelength/nm Colour 380 Violet 400 Violet -deep blue 436 Deep blue 440 Deep blue 462 Light blue 490 Very light blue512 Bluish/turquoise 614 Greenish gold/yellow 670 Light orange 710Orange 753 Deep orange 802 Light orange Crimson

Three samples were photographed under different lighting conditions. Itis evident that the degree of variation in hue with lighting conditionand viewing angle is dependent on the sample selected—some colors (e.g.a silver-blue sample) vary more than others (e.g. a mid-blue sample).The strongest color change is observed on a sunny day while viewing themirror-reflection of the sunlight.

The following samples were photographed:

1. Mid-blue: seven-layer “plasma” coating; quarter-wave stack, centredat 430 nm.

2. Silver-blue: seven-layer “plasma” coating; quarter-wave stack.

3. Silver: metal+dielectric vacuum coating.

These three examples were chosen since they provide a range of thecoating types and colors that appear not to change much to the unaidedeye (e.g. samples 1 and 3) and that do change most noticeably with angleof viewing/incident light (sample 2).

The photographs did not capture the visual effect—they appear flat,whereas the samples look three-dimensional and mesmeric. Also, thephotographs did not capture the precise hues—the mid-blue sample, forexample, appeared a much deeper blue in the photographs, whereas to theeye they appeared a mid, “electric” blue (at normal incidence). However,the photographs did capture the change in hue under differentlight/viewing conditions.

The light conditions chosen represent the (near) extremes of what can beencountered in an average day (excluding specialized room lights). Theangle at which the photographs were taken where 0 degrees, 45 degreesand 75 degrees. Where the sun is not directly overhead (as it is atmidday), photographs were taken from within two planes: (i) that of thesun's path in the sky, and (ii) that perpendicular to (i). For (i),photographs were taken facing into the sun (i.e. capturing themirror-reflection) and with the sun behind the camera (i.e. capturingthe retro-reflection). In the near-diffuse conditions, the samples werephotographed in a room with white walls and ceiling, at midday, underovercast conditions, with sunlight only illuminating the room from alarge, open door (only weak shadows were seen in the room, indicatingnear-diffuse conditions, with minor directional light). All photographswere taken in Sardinia on 28 and 29 Jul. 2015 (i.e. near mid-summer).

The results of the photographs were as follows:

1. Sample 1 (mid-blue) did not change significantly in hue within anglesof viewing of at least 75 degrees around the normal (i.e. a 150 degreeviewing cone). The only exception to this is when it was viewed inmirror-reflection, in bright sunlight, but this condition is quiterestricted.

2. Sample 2 (silver-blue) varied in hue considerably under the differentviewing conditions, although not so much in near-diffuse light.

3. Sample 3 (silver) lay somewhere between the effect of Samples 1 and2, appearing silver in hue with angles of viewing of at least 45 degreesaround the normal (i.e. a 90 degree viewing cone), then becomingbluish-silver in appearance.

These results indicate that the preferred optical coating structuresappear more consistent in hue with viewing conditions for some colorsthan others. For some commercial applications, color consistency will beimportant and therefore favour certain hues of optical coatingstructures, although a change in hue may be desirable for otherapplications.

For mid-blue, for example, a bright, mesmeric color can be achieved thatchanges little with most viewing or light conditions. This can beachieved for other hues, too.

The materials of the thin-film layers, particularly the innermost andoutermost layers, will affect the resilience of the optical coatingstructure to everyday use. Oxide materials such as SiO₂, ZrO₂ and TiO₂,at least, are known to be particularly “tough”, resilient materials. Inaddition, adhesion of the layers to the substrates is greater than thatfor flat profiles, since the surface topography helps to improveadhesion.

Samples of the optical coating structure were also examined for theirresilience to general handling. Scanning electron micrographs of twoindentation-type base structures coated with three-layers (blue) in asputter coating machine were examined. A few black regions wereidentified as areas that had been damaged during handling. However, nodifference to the overall colored appearance of the device was observed.

Samples were also examined to see the effect of fingerprints.Fingerprints cause grease layers on the surface, which appear, under themicroscope, as tiny oil patches, creating interference colors. Severalfingerprints can leave around 1% coverage in such “oil”. It was foundthat these can be cleaned off with acetone, for example, without harmingthe optical coating structure.

Scratch tests and a tape adhesion test, known as “mil spec tests” wereconducted on the same three-layer coated samples (indentation-type basestructures) as follows. Note that these samples were made by ordinarysputter coating (not plasma coating), i.e. at high temperature;considerable outgassing from the acrylic substrate would have occurredat the high temperatures used, which probably led to relatively pooradhesion of the films to the substrate (i.e. plasma coated samples wouldhave performed better in the scratch test).

The tests comprised a mild abrasion test, a severe abrasion test and atape adhesion test. The mild abrasion test was performed involving 50“rubs” of a cheesecloth abrader on the optical coating structure sample.The samples passed this test. The severe abrasion test involved 20“rubs” of a rubber abrader impregnated with grit. Damage to thereflector was observed at this high level of abrasion. The tape adhesiontest involved pressing adhesive tape against the optical coatingstructure and ripping it off the surface. No removal of any part of thereflector was observed meaning that the optical coating structure passedthis test. Additional coatings of up to 9 layers, made using a plasmacoating system, also passed the adhesion (“tape”) test.

FIGS. 14A and 14B illustrate scanning electron micrographs showingdamage to a reflector coating of sample optical coating structuresduring the mild and severe abrasion tests respectively. Both images aretaken of a central region of the particular sample where the abrasionwas most intense. In the micrographs, the reflector appears white andthe base layer appears dark grey where the coating has been rubbed off.The tests established that the optical coating structure samples couldstill provide good color effects despite small amounts of damage beingobserved during the mild abrasion tests. When much more substantialabrasion was applied it was found that only small amounts of reflectorremained left on the base layer and these were insufficient to provide adesired color effect. This may be a consequence of the material of thesubstrate, which affects the level of attachment of the thin-filmlayers.

To help provide abrasion resistance for the optical coating structure, aprotective, transparent coating can be applied to the reflector.Parylene, for example, adheres extremely well to the oxide layers of aquarter wave stack and can provide a good level of protection. Itgenerally does not affect the perceived color (hue) of the opticalcoating structure, but it does alter the matt effect. At thicker levels,the outermost surface of the Parylene coating begins to flatten out,imparting mirror-like reflections of white light (it becomes shiny).This may or may not be desirable. Such coatings may also provideresilience to fingerprints and smear marks.

In practice, the material of the substrate (which might be of, forexample, acrylic or other plastics) may be softer than the materialsthat the optical coating structure is likely to encounter during dailyuse (for example, keys or coins), and deformation of the profileelements may result in some loss of the reflective properties of thestructure. While the oxide layers of the reflector can provide somescratch resistance, in some cases it may be desirable to apply anadditional harder surface layer to try to minimise further any scratchdamage.

Optical coating structure samples, made using reflectors consisting ofseven layers of TiO₂ and SiO₂ and also metal plus dielectric (two)layers, applied onto 3 mm thick acrylic base layers provided withprofile elements, were heated on a hot plate to 70 degrees Centigrade.No color change was observed, indicating that these optical coatingstructures can be used on a variety of day-to-day objects to providepermanent color without degradation during normal use.

FIG. 15 shows the emission spectrum of a (white) lamp used in thereflectivity measurements of FIGS. 16A to 20B. The x-axis showswavelength in nm while the y-axis shows reflectivity in arbitrary units.A spectrometer using the lamp was used to take peak wavelength andreflectance measurements under white, directional light at differentangles of incidence/reflection (the mirror-reflection angle wasmeasured). The measured spectra were normalized against the emissionspectrum of the lamp.

FIG. 16A shows a series of five reflection spectrums received from ablue optical coating structure having indentation-type profile elements,each from a different angle of reflection; angle 1 is 12°, angle 2 is20°, angle 3 is 22.5°, angle 4 is 29° and angle 5 is 35°. FIG. 16B showstwo reflection spectrums received from a blue glass optical coatingstructure having indentation-type profile elements; where angle 1 is11°, angle 2 is 21°.

FIGS. 17A through to 20B show reflection spectrums received from anumber of different optical coating structures when illuminated with thewhite lamp in the spectrometer. FIGS. 17A and 17B show the reflectionspectrums for a blue optical coating structure having indentation-typeprofile elements at 45° and at near-normal respectively. FIGS. 18A and18B show reflection spectrums from a crimson optical coating structurehaving indentation-type profile elements at 45° and at near-normalrespectively. FIGS. 19A and 19B show reflection spectrums from a glassoptical coating structure having indentation-type profile elements at45° and at near-normal respectively. FIGS. 20A and 20B show reflectionspectrums from a blue optical coating structure having projection-typeprofile elements at 45° and at near-normal respectively. In all cases,the units for reflection are arbitrary since the amplitude would dependon many factors such as the brightness of the lamp and the distance awayfrom the sample.

For all the tested samples, the reflection spectrums illustrate how thedifferent angles of incidence and reflection can change the wavelengthof the reflection, shifting the reflection to shorter wavelengths as theangle of incidence/reflection becomes steeper. Despite this change,however, when the samples were observed with the naked eye, due toglobal averaging of the eye, the observer tends to see a strong, brightreflection of a single color from a wide range of directions—the samplesappearing intense but not shiny, providing a mesmerizingly deep,luxuriously matt effect.

The invention claimed is:
 1. An optical coating structure that whenapplied to a surface of an object imparts a structural color to theobject wherein the color remains substantially the same to an observerover a broad range of viewing angles, the optical coating structurecomprising: a base layer; a multilayer reflector on the base layer toimpart the color; and profile elements on the base layer under thereflector wherein the reflector conforms to the surface profile of theprofile elements, the profile elements having a width and length whichare each in the range of 5 to 500 μm in size, and being arranged innon-periodic manner such that the profile elements are configured toavoid diffraction effects, wherein the profile elements are shaped sothat greater than 50% of the surface area of the reflector isapproximately normal to the incoming light when the light is incident atthe normal to the plane of the surface and shaped so that greater than30% of the surface area of the reflector is approximately normal to theincoming light when the light is incident at an angle of up to 30degrees, wherein the profile elements reduce the multilayer reflector'snormal property of color change with changing viewing angle.
 2. Anoptical coating structure according to claim 1, wherein the base layeris a black colored material, or a transparent material to visible light.3. An optical coating structure according to claim 1, wherein theprofile elements have a height which is in the range of 0.1 to 50 μm. 4.An optical coating structure according to claim 1, wherein the profileelements are an array of overlapping bumps, or are a non-periodic arrayof indentations.
 5. An optical coating structure according to claim 1,wherein the multilayer reflector comprises layers of higher and lowerrefractive index materials, wherein the multilayer reflector consists ofless than 20 layers, wherein the multilayer reflector is configured toproduce a reflection having a peak wavelength in the range of 350 to 800nm, and wherein there is no break in the continuity of the layers of thereflector.
 6. An optical coating structure according to claim 1, whereinthe width and/or length of at least 50% of the profile elements is 10 to100 times the thickness of the reflector; wherein the average periodbetween adjacent profile structures is between 5 and 100 μm, and whereinat least 60% by area of the profile elements have a width and lengthwhich are each in the range of 5 to 100 μm in size.
 7. An opticalcoating structure according to claim 1, wherein the profile elementshave a ratio of height and length dimensions between 1:2 and 1:100. 8.An optical coating structure according to claim 1, wherein the structurecomprises a protection layer on the upper surface of the reflector,which is a layer of SiO₂, about 1 μm thick, or is a layer ofpoly(p-xylylene) polymer.
 9. An optical coating structure according toclaim 1, wherein the structure comprises an additional coating on theupper surface of the multilayer reflector which is configured to shiftincident light towards a normal of the multilayer reflector.
 10. Anoptical coating structure according to claim 9, wherein the additionalcoating comprises a material that has been cured after application, andwherein the additional coating comprises an acrylic material.
 11. Anoptical coating structure according to claim 1, wherein the base layeris an acrylic material.
 12. An object comprising the optical coatingstructure of claim 1, wherein the optical coating structure isincorporated on a surface of the object to impart a structural color tothe object, and wherein more than 50% of a surface of the objectincorporates the optical coating structure.
 13. An object accordingly toclaim 12, wherein the object is a housing of a portable electricaldevice or wherein the object is a sheet of flexible material for use inarticles of manufacture.
 14. A method of forming an optical coatingstructure that when applied to a surface of an object imparts astructural color to the object, wherein the color remains substantiallythe same to an observer over a broad range of viewing angles, the methodcomprising: providing a base layer, the base layer having profileelements thereon, the profile elements having a width and length whichare each in the range of 5 to 500 μm in size, and being arranged in anon-periodic manner such that the profile elements are configured toavoid diffraction effects, wherein the profile elements are shaped sothat greater than 50% of the surface area of the reflector isapproximately normal to the incoming light when the light is incident atthe normal to the plane of the surface and shaped so that greater than30% of the surface area of the reflector is approximately normal to theincoming light when the light is incident at an angle of up to 30degrees; and depositing a multilayer reflector on the base layer toimpart the color, wherein the reflector conforms to the surface profileof the profile elements and wherein the profile elements reduce themultilayer reflector's normal property of color change with changingviewing angle.
 15. A method as claimed in claim 14, wherein anacid-etching process is used to form a mold for forming the profileelements.
 16. A method as claimed in claim 14, wherein plasma coatingprocess is used to deposit multiple layers to form the reflector, andwherein the plasma coating process deposits between 2 and 8 layers ofalternating dielectric materials.
 17. A method as claimed in claim 14,including a step of depositing a protection layer on to a surface of thereflector.
 18. A method as claimed in claim 14, including a step ofdepositing an additional coating on to a surface of the multilayerreflector, the material of the additional coating being selected toshift incident light towards a normal of the multilayer reflector.
 19. Amethod as claimed in claim 14, wherein the method includes the step ofbending the base layer with profile elements already formed thereon.